1. Field of the Invention
The invention relates in general to solid freeform fabrication and, in particular, to a feedback system for determining the deposition rate of a selective deposition modeling apparatus so as to optimize the deposition rate to increase build speed and reduce the generation of waste.
2. Description of the Prior Art
In selective deposition modeling, herein referred to as xe2x80x9cSDM,xe2x80x9d complex parts are produced from a modeling material in an additive fashion as opposed to traditional fabrication techniques, which are generally subtractive in nature. For example, in traditional fabrication techniques material is removed by machining operations or shaped in a die or mold to near net shape and then trimmed. In contrast, additive fabrication techniques such as SDM incrementally add portions of a build material to targeted locations, layer by layer, in order to build a complex part.
In SDM, a solid modeling material is physically deposited in successive fashion to form an object. In one type of SDM technology the solid modeling material is extruded as a continuous filament through a resistively heated nozzle. In yet another type of SDM technology, the solid modeling material is jetted or dropped in discrete droplets in order to build up a part. Often, a thermoplastic material having a low-melting point is used as the solid modeling material, which is delivered through a jetting system such as those used in ink-jet printers. One type of SDM process utilizing ink-jet print heads is described, for example, in U.S. Pat. No. 5,555,176 to Menhennett, et al. Another type of SDM process which extrudes a bead of material to build a part is described in, for example, U.S. Pat. No. 5,303,141 to Batchelder et al. Still other SDM processes dispense two different solidifiable materials, such as the SDM process described in U.S. Pat. No. 5,136,515 to Helinski. In addition, other SDM processes dispense sintered powders, pastes, and liquids, as described respectively in U.S. Pat. No. 5,017,753 to Deckard, U.S. Pat. No. 6,110,409 to Allanic et al., and U.S. Pat. No. 5,236,637 to Hull.
Although SDM methods have many advantages compared to conventional fabrication methods, they also have unique problems that are rooted in the layer by layer building process. One common problem in the layer by layer building process results from the dimensional variability inherent in the building of each layer. These dimensional inaccuracies result from numerous phenomena, such as the accumulated effects of drop volume variation, thermal distortion, errors in deposition, and the like. In addition, the type of geometrical configurations being formed can also influence these inaccuracies, such as the production of web or branching supports. Also, a weakened or blocked dispensing jet will contribute to these inaccuracies. If unchecked, these tolerances can accumulate throughout the part as it is built up in height layer by layer. As the thickness of layers is reduced in order to achieve greater surface resolution, the accumulated effects of these undesirable tolerances can substantially distort the resultant part. Thus, most SDM processes require some method to dimensionally normalize or smooth the top working surface of the part while it is being built.
Generally, most dimensional normalization methods physically adjust the vertical height of the part by smoothing or leveling the build material deposited in the layers. This commonly produces waste material. Such systems are open loop systems that provide no active feedback to compensate the build rate of the layer. For example, in one open loop approach, each layer of build material is dispensed at a thickness that is to greater than a desired thickness of the layer, and then a normalizing device is activated to remove the excess build material to achieve the desired thickness. Although distortions between the layers are eliminated, waste material is generated.
An example of the excess build material approach is found in U.S. Pat. No. 5,943,235 to Earl et al., wherein a pre-heated rotating planarizer is provided to normalize each layer. Under this approach, after a layer of build material has been deposited by the SDM apparatus in excess of the desired thickness, the pre-heated cylindrical roller (planarizer) is passed over the deposited material to establish the desired thickness. The rolling planarizer locally melts a portion of the build material of the layer. Some of the material adheres to the surface of the planarizer as it rolls, and a wiping or scraping device such as a blade peels or skives off the excess build material from the planarizer. The excess build material is accumulated by the planarizer as waste material.
Other dimensional normalizing systems that generate waste material are found in U.S. Pat. No. 5,859,775 to Barlage, III et al. and U.S. Pat. No. 5,572,431 to Brown et al. Under these approaches, a heated body is selectively driven across the dispensed build material in response to a sensed deviation in order to melt and displace the build material. The excess material is then sucked off by a vacuum source connected to the heated body. Thus, waste material is commonly generated during most dimensional normalization processes used in SDM.
In all SDM systems it is desirable to reduce the time required to build an object. However, dispensing material in an amount in excess of that needed undesirably increases build time. For example, if build material is dispensed in excess of the Id desired layer thickness by about 35%, then about 35% of the time spent dispensing material does not contribute to forming the resultant part. Further, 35% of the material will be waste at the end of the build process. Thus, it would be desirable to reduce the amount of excess material that is dispensed in order to increase build speed and decrease waste.
However, many dispensing devices such as ink jet print heads tend to degrade over time due to thermal degradation. This thermal degradation often results in a reduction in the quantity of material being dispensed, often referred to as drop volume. Over time this degradation can be significant, particularly for print heads that are supplied with a constant firing voltage. For example, if a print head were initially set to dispense in excess of 35% when new, it may only dispense in excess of 5% at a later time after thermal degradation has set in. Thus, a substantial reduction in build time and generation of waste material can be realized by optimizing the deposition rate of the print head as thermal degradation sets in. However, developing such a system to optimize the deposition rate has proven problematic.
Active surface imaging systems have been proposed to normalize the surface of each layer. Such systems actively monitor the surface condition of any given layer by collecting image data that is processed to provide feedback data that can be used to selectively dispense additional material in low areas to form a uniform layer. One such system is disclosed in U.S. Ser. No. 09/779,355 to Kerekes, filed on Feb. 8, 2001. Although such an active closed loop imaging system can eliminate the use of a planarizer, it may introduce additional complexities to the apparatus that may impact reliability and cost. If a planarizer system can be optimized in conjunction with adjusting to the deposition rate of the print head, there may be no need to introduce a complex active surface scanning system in an SDM apparatus.
Thus, there is a need to provide an SDM process capable of dimensionally normalizing layers of a three-dimensional object by generating a minimal amount of waste material. There is also a need to decrease build time by reducing the amount of time spent depositing excess build material for each layer. These and other difficulties of the prior art have been overcome according to the present invention.
The present invention provides its benefits across a broad spectrum of three-dimensional building processes. While the description which follows hereinafter is meant to be representative of a number of such applications, it is not exhaustive. As will be understood, the basic apparatus and methods taught herein can be readily adapted to many uses. It is intended that this specification and the claims appended hereto be accorded a breadth in keeping with the scope and spirit of the invention being disclosed despite what might appear to be limiting language imposed by the requirements of referring to the specific examples disclosed.
It is one aspect of the present invention to form a three-dimensional object with an SDM apparatus at the highest build speeds possible.
It is another aspect of the present invention to dimensionally normalize the layers of a three-dimensional object by an SDM apparatus by generating a minimal amount of waste material.
It is still yet another aspect of the present invention to utilize feedback data indicative of the amount of excess build material being deposited in each layer when forming a three-dimensional object to optimally adjust the deposition rate of an SDM apparatus.
It is a feature of the present invention to determine the amount of excess build material being deposited by measuring the amount of power that is utilized when normalizing the layers.
It is an advantage of the present invention that build time can be optimized when using a planarizer to normalize the layers of an object formed by an SDM apparatus.
It is another advantage of the present invention that the build speed of the SDM apparatus can be optimized throughout the life span of the dispensing head.
It is yet another advantage of the present invention that a closed loop system can actively adjust the deposition rate of an SDM apparatus in response to feedback data obtained from measuring the power used in dimensionally normalizing a layer.
These and other aspects, features, and advantages are achieved/attained in the method and apparatus of the present invention that employs a sensor mounted on the planarizer that provides feedback data indicative of the height of the layer of material being normalized. The feedback data is then utilized to adjust the overall firing voltage supplied to the print head to either increase or decrease the deposition rate, as needed. Alternatively, the desired layer thickness can be increased or decreased in response to the feedback data, if desired.